It is a well known fact that all the industrial processes used nowadays for removing iron impurities from ores and particularly for removing iron impurities from silica sand, particularly when silica sand is to be used for glass manufacture, derive from a very well known and old concept which comprises transforming the iron oxide and other iron insoluble impurities, into soluble iron salts, and that this solubilization of the iron impurities has always been effected by treatment with a chemical agent, sulfuric acid being the preferred chemical agent in the so called SAT (Sulfuric Acid Treatment), which is widely used in all the modern installations for treating silica sand to be used for the manufacture of glass.
Thus, for example, Knowles et al, in U.S. Pat. No. 2,306,021, describe the said principle for the first time, in their application dated 1940, and describe a process of removing iron oxide from natural silicon dioxide sand, which includes converting the iron oxide into a soluble iron salt by the action of an acid solution containing sulfuric acid and sodium chloride.
One other achievement in the art is the process described and claimed by Adams et al in U.S. Pat. No. 2,891,844, wherein the purification of silica sands is accomplished by the reduction of the surface iron oxide content (which stains the silica sand particles and is difficultly releasable therefrom), which reduction is achieved only by up to about 15% of the total iron oxide content, and comprises subjecting the sand to treatment with a hot aqueous solution containing sulfuric acid and a soluble inorganic chloride, said process being carried out in the presence of metallic copper having a large surface area so as to give an ample metallic surface contact for the sand and the solution during the purification treatment.
In turn, Slade et al, in U.S. Pat. No. 3,914,385, describe a process for purifying contaminated sand, which comprises firstly removing by froth flotation a plurality of contaminants such as the kaolin clays and siderite, then treating the resulting partially purified sand with sulfuric acid for a time and at a temperature sufficient to reduce the iron oxide content thereof, and finally filtering the aqueous sulfuric acid leach liquor from the sand and drying the sand.
These processes, however, have proven to be absolutely uncapable of removing the strongly adhered stains of iron impurities in certain mineral ores, particularly in certain types of sand, whereby it has been materially impossible to obtain sufficiently pure minerals to be used for certain purposes, whereby this type of processes have been used only to partially purify said minerals, without accomplishing a purification sufficiently high for certain special uses of said ores.
In an attempt to solve the above mentioned problem, other workers have proposed certain processes which supposedly increase the degree of purification of the ores from iron impurities, and thus, Sturgeon et al in U.S. Pat. No. 2,592,973, suggest a process for reducing iron impurities in minerals which consists in treating the minerals with an aqueous solution which is free from fluorine ions and oxalic acid but in which the dissolved constituents consist essentially of metallic salts, one of which is selected from titanous sulphate and titanous chloride and one other is selected from sodium sulfate and sodium chloride, and recovering the purified mineral product.
To the knowledge of applicant, this process never resulted in a complete purification of the mineral ores and, on the other hand, the use of titanium salts renders this process absolutely uneconomical, particularly for purifying sand which is a low economical density product, that is, has a relatively low price, and for which titanium compounds are not justifiable. Therefore, this process has been disregarded for any commercial exploitation, in view of its relatively high cost and poor economical results.
One other suggested process is the process of Gross, described in U.S. Pat. No. 2,952,516, which discloses removing iron from silica sand by the steps of adding hydrochloric acid and fluosilicic acid as such to said sand to form an aqueous slurry, separating the sand from the liquid and subsequently washing the sand with water until substantially free of acid. This process, while removing efficiently the iron stains, has the disadvantage that the combination of hydrochloric and fluosilicic acids dissolves to a great extent the silica particles and, therefore, the loss of wanted mineral with the unwanted impurities is so great that this process must also be regarded as absolutely uneconomical and of not practical use whatsoever.
Chubb, in U.S. Pat. No. 3,297,402, in turn, discloses a method of decreasing the quantity of iron sulfide contaminants in silica sand, which comprises the step of leaching the sand with a hypochlorite, but this process is restricted only to remove iron sulfides and not iron oxide, and it is a well known fact that the main contaminant of mineral ores and particularly of silica sand, are iron oxides. Therefore, this process must be discarded as an efficient process for purifying sand, particularly when the latter is to be used for the manufacture of glass.
One other process is described by Segrove in U.S. Pat. No. 3,050,364, and comprises contacting the sand, particularly when the latter is to be used for glass making, at a temperature below 250.degree. C. and in the dry state, with substantially dry gaseous hydrogen chloride until change in color of the sand grains substantially ceases, which indicates that the reaction is substantially complete between the hydrogen chloride and the iron impurities, and then the sand is washed with water to remove the solubilized impurities. While this process might be regarded as an efficient process for removing iron stains from particles of silica sand, it is also highly inconvenient, mainly in view of the following facts: firstly, the dry hydrogen chloride tends to react with the silica particles and therefore partially dissolves the same when the sand is washed with water and, secondly and more important, it requires of very complex installations which must be made of a gas-proof material and must be capable of handling gaseous hydrogen chloride in the dry state, and no doubt this fact complicates the erection of a plant and increases the cost of the process so greatly, that the same may not be regarded as a practical method for purifying mineral ores and particularly sand for glass making purposes.
Finally, Bowdish, in U.S. Pat. No. 4,042,671, proposes and describes a special leaching method for purifying sand and other particulate materials, which comprises leaching iron and other metallic compounds from sand by a series of aqueous solutions of differing compositions, differing in concentrations of leaching component in the form of an acid, and in which the aqueous solutions are fed to the top of the mass of the sand and are permitted to drain through gravitationally with appropriate periods of time between solutions for reaction, said solutions of differing compositions forming series which begin with solutions low in concentration of the leaching component and continue with solutions of higher concentration thereof until the solution of highest strength has been reached at about midway, and then reversing the order by reducing the concentration of the solutions until water is reached. This method is extremely complicated to be controlled adequately and has not proven to give any better results than the common process used presently in all the practical commercial installations for purifying sands, whereby said process has not gained sufficient popularity in view of its uneconomical characteristics.
The modern industrial plants for purifying silica sand, have acquired a technology derived from the accomplishments of the above workers, inter alia, and have developed the very well known process of purifying silica sand which essentially comprises grinding the sand or ore, washing and desliming the ground ore to remove the major part of the clay-type binder, attrition-scrubbing the deslimed particles to release further amounts of binder therefrom, washing and desliming to remove said binder, then treating the particles in suspension with sulfuric acid in order to convert the iron impurities into water soluble iron compounds, and finally washing and desliming with water the chemically treated particles in order to remove as much as possible of the iron impurities from the ore. However, while this process has proven to be the best of all the above described processes, it still has the enormous disadvantage that it is absolutely unable to remove and release from the ore particles (particularly sand) the iron impurities which are in the form of strongly adhered stains on each individual particle, and therefore the sand or other ore obtained by this process, cannot be regarded as a highly purified ore, because said strongly adhered stains of impurities resist the chemical treatment and are not released from the individual particles by said chemical treatment followed by the washing and desliming operations.
Therefore, all the prior art processes for removing iron impurities which are in the form of strongly adhered stains on particles of ore, have proven to be absolutely unable to accomplish this goal, with the consequent disadvantages and problems caused to the plants which use this ores as starting materials for carrying out other production operations.